Evgeniy M. Myshakin

Quantum energy flow and the kinetics of water shuttling between hydrogen bonding sites on trans-formanilide.

A potential energy surface for trans-formanilide (TFA)-H2O is calculated and applied to study energy flow in the complex as well as the kinetics of water shuttling between hydrogen bonding sites on TFA. In addition to the previously identified H2O-TFA(C[Double Bond]O) and H2O-TFA(NH) minima, with the water monomer bound to the C[Double Bond]O and NH groups, respectively, the new surface reveals a second local minimum with the water bound to the C[Double Bond]O group, and which lies energetically 310 cm(-1) above the previously identified H2O-TFA(C[Double Bond]O) global minimum. On this surface, the energy barrier for water shuttling from H2O-TFA(C[Double Bond]O) global minimum to H2O-TFA(N-H) is 984 cm(-1), consistent with the experimental bounds of 796 and 988 cm(-1) [J. R. Clarkson et al. Science 307, 1443 (2005)]. The ergodicity threshold of TFA is calculated to be 1450 cm(-1); for the TFA-H2O complex, the coupling to the water molecule is found to lower the ergodicity threshold to below the isomerization barrier. Energy transfer between the activated complex and the vibrational modes of TFA is calculated to be sufficiently rapid that the Rice-Ramsperger-Kassel-Marcus (RRKM) theory does not overestimate the rate of water shuttling. The possibility that the rate constant for water shuttling is higher than the RRKM theory estimate is discussed in light of the relatively high energy of the ergodicity threshold calculated for TFA.

Theoretical and infrared spectroscopic investigation of the O2−∙benzene and O4−∙benzene complexes

The infrared spectra of the O(2) (-).benzene and O(4) (-).benzene complexes are determined by means of Ar predissociation spectroscopy. Several transitions due to CH stretch fundamentals and various combination bands are observed in the 2700-3100 cm(-1) region. The experimental results are interpreted with the aid of electronic structure calculations. A comparison of the calculated and experimental spectra reveals that the spectrum of O(2) (-).benzene most likely arises from an isomer where the superoxide molecule binds preferentially to one CH group of benzene. In contrast, the spectrum of O(4) (-).benzene yields a CH pattern remarkably similar to that displayed by the C(2nu) X(-).benzene (X=halogen) complexes, consistent with a structure with two CH groups equally involved in the bonding. The lower energy vibrational fundamental transitions of the O(4) (-) anion are recovered with a slight redshift in the O(4) (-).benzene spectrum, establishing that this charge-delocalized dimer ion retains its identity upon complexation.

Monte Carlo and Molecular Dynamics Simulations of Clay Mineral Systems

This chapter is focused on reviewing molecular dynamics and Monte Carlo simulations of greenhouse gases’ interactions with swelling clay minerals. This chapter unfolds with the results of simulations on stepwise expansion of interlayer in hydrated montmorillonite. Next, an overview of the simulation data on carbon dioxide intercalation in clays is given with respect to structural changes, transport properties, thermodynamics, spectroscopic characteristics, sorption behavior at the basal clay surfaces, and surface wettability changes in CO2-brine-mineral systems. Effects of the chemical nature of interlayer ions, as well as charge density and its distribution within clay layers on carbon dioxide/water intercalation and interaction with clay surfaces, are discussed. Then, results of methane interaction with hydrated swelling clays are presented. The discussion is centered around the formation of gas hydrate phase in the interlayer under suitable pressure and temperature conditions. Dynamic nature of hydrate cages encapsulating methane molecules is considered together with a mechanism of their formation in interlayer. A shift of the equilibrium pressure and temperature conditions in comparison with bulk phase is attributed to distortion of hydrate lattice in clay and to finite pore space. Finally, intercalation of the carbon dioxide/methane molecules in interlayer is reviewed through competitive adsorption of the binary mixture on clay surfaces.

Advances in Molecular Simulation Studies of Clay Minerals

The unique structure and behavior of swelling clay minerals, as observed in the laboratory and in the environment, present a challenge in understanding of the molecular details associated with these minerals. The chapter introduces the essence of classical methods involving empirically derived potential energy expressions that allow simulation of periodic cells representing bulk and interfacial clay mineral systems. The classical models provide the simulation and analysis of many thousands to more than a million atoms for evaluating structures, adsorption, diffusion, intercalation, physical, and other properties. Quantum chemical calculations, including molecular orbital methods and density functional theory, optimize the configuration of electrons about atoms from first principles, but require significant computational cost to examine many of the important topics in clay mineralogy. Molecular simulation methods such as energy minimization, molecular dynamics, Monte Carlo techniques, vibrational analysis, thermodynamics calculations, transition state analysis, and a variety of related computational methods are utilized to improve our understanding of clay minerals, and to better interpret traditional characterization and spectroscopic methods. An example showing the use of molecular simulation for clay minerals is presented for the process of montmorillonite’s swelling as a function of interlayer water.

Experimental Studies: Clay Swelling

The best-known characteristic of clay is a dramatic change in its morphological and geomechanical properties: from hard, dense, and brittle upon drying or firing to soft, pliable, and swelling upon exposure to water. Chemical properties of the 1:1 and 2:1 clay minerals are significantly different, which is mainly related to the bonds between individual layers. The interlayer environment is determined by the chemical nature of clay layers, the layer charge, interlayer cations, and water molecules forming hydration shells around the cations and H-bonding with clay surfaces. Mechanisms of water sorption and cluster organization are electrochemical in nature and fundamental to the swelling process. Some researchers also observed irreversible CO2-induced swelling with smectite in 1–2 W hydration state, but the others reported only shrinking attributed to drying effects of high-pressure CO2, for the cation-exchanged smectite with partly filled second hydration layer. The current interpretation of swelling phenomena evolves rapidly, following advances in experimental techniques and Monte Carlo and MD simulations of the structured fluid behavior. MD simulations show that the interlayer molecules do not organize themselves in a strictly tilted or strictly parallel to the surface configuration, which may result in fairly steep but gradual rather than stepwise increase in the basal spacing as the interlayer is filled with the solvent molecules. The magnitude of swelling hysteresis varies with the hydration energy of the interlayer cations and is generally more pronounced for vermiculite than montmorillonite.